Balanced slot directional coupler



Dec. 17, 19.57 L. A. KURTZ 2,817,063

BALANCED SLOT DIRECTIONAL COUPLER Filed Sept. 12, 1952 VIIIIIIIIIIII/lllllllllll United States Patent BALANCED SLOT DIRECTIONAL COUPLER Louis A. Kurtz, Los Angeles, Calif., assignor, by ymesne assignments, to Hughes Aircraft Company, a corpora- `:tion of Delaware Application September 12, 1952, Serial No. 309,262

1 Claim. (Cl.,333-10) This invention relates to waveguides lfor microwave electromagnetic energy, and more particularly to a directional coupler adapted to couple circularly or elliptically polarized microwaves between a pair of waveguide sections.

A directional coupler is a device for abstracting electromagnetic energy from a transmission line in such a manner that energy `propagating in one direction is sepparated from energy propagating in the reverse direction. Usually such a device comprises a main transmission line, and an auxiliary transmission line coupled thereto by means of two equal coupling links, spaced -apar't one-quarter wavelength of the electromagnetic energy propagating along the transmission line. Because of the spacing of these coupling links, energy propagating in one direction, in the main transmission line, also prolpagates in the same direction in the auxiliary transmismission line, and no other. For this reason, a directional coupler is `capable of separating transmitted and reected energy. In addition, operation similar to that of a hy- `brid junction is obtainable, provided that one-half the energy in the main transmission line is coupled to the auxiliary transmission line.

A directional coupler may comprise various Atypes of transmission line such as a coaxial line, parallel wire line,

or waveguide, since its mode of operation is independent transmission, commonly known as waveguide wavelength. A plurality of such pairs may be included -in order to achieve the desired amount of coupling.

In the past ,the coupling slots in the waveguide sections have also been arranged in a different manner. For

example, a group of two slots, disposed in the form of a T, has been used in place of a pair of slots spaced onelquarter wavelength apart. The particular spacial arrangement of the coupling slots which is most suitable 'for any given application of a conventional directional coupler, is determined principally by the amount of coupling required, and the bandwidth of the energy to be coupled, provided, of course, that a directional type of coupling is accomplished.

It is to be noted that directional couplers of conventional design, for operation at microwave frequencies, utilize rectangular waveguides having transverse dimensions substantially of the ratio 2:1. Therefore, itris readily apparent that prior art devices of this type are *.-not designed for operation with two orthogonal modes which 'produce a circularly or elliptically polarized wave.

l2,817,063 :PatentedY Dec. 17, 1.957

Nor would it be Apossible to obtain .such Aoperation -bysubstituting a diiferent type of waveguide. Were this `substitutionimade, either one mode would be suppressed-or `undesired .modes excited, owing to the special arrangements of the slots hitherto practiced.

More particularly, a slot-in `a waveguide is excited 'when there are components of current whose direction of ow is transverse to the longitudinal axis of the slot. A slot which is located off the longitudinal centerline of a waveguide wall intercepts such transverse current in either a TEM or TEN mode, that is, ineither of two orthogonal modes wherein the electric vectors, 1E, extend vin a horizontal and vertical direction, respectively. Consequently, a slot, which is so located, isrexcited by either of Vthese modes.

However, when such a slot is excited by either a TEM or TEN, mode, both of these modes are excited by the slot itself. Therefore, ,it this slot ,is used to couple a TEM mode, it will excite not only the TEM, but also the T1510 mode Similarly, if this slot is used to couple a TEN mode, it will also lexcite TEM and 'Tl-310 modes. If this slot is used to couple both TEM and TEM, modes simultaneously, that is for example, a-circularly polarized wave, it will excite TEM and TEM modes of different rela- .tive amplitudes, resulting in a wave having elliptical v polarization.

Recent developments in the related arts emphasize the utility `of a directional coupler capable of operation with two orthogonal modes.

According to the present invention, therefore, a directional coupler is provided which is capable of operation with two orthogonal modes, propagating either simultaneously, or alternatively. Thus, any kind of polarization, including circular or elliptical polarization, .can be directionally coupled by means yof this device, and in addition, any amount of coupling can be provided.

In order to 'accomplish this result, two parallel iand contiguous sections of waveguide, having a common dividing wall, are intercoupled by means of 'one or more siot groups, each consisting of four slots, cut in the common wall. The slots, lcomprising a rst pair, are symmetricallydisposed about the longitudinal centerline fof the 4common wall, so that the line intersecting their centers is perpendicular thereto. The slots :comprising the second pair are similarly located with respect to the centerline, the lines intersecting the centers of the pairsbei'ng separated by a distance of one-quarter waveguide wavelength. A plurality of such `groups may be arrayed lalong the common wall according 'to the amount of coupling required.

The novel operation of the directional coupler herein described, which makes directionalcoupling possible -ir respective of polarization, arises from the symmetrical, or balanced arrangement of the individual slots comprising a given pair. Because of this symmetry, thepropagating modes in the auxiliary waveguide are identical with the propagating modes in the main waveguide. This means that a TEM mode in one waveguide appears only as a TEM mode in the other waveguide, or, alternatively, a TEM mode appears only as a TEM) mode.

Accordingly, it is an object of this invention to pro vide a directional coupler which is capable of operation with circularly or elliptically polarized waves.

It is another object of this invention to providev a directional coupler which is capable of operation with either one of two orthogonal modes in thefalternative. y

It is a further object to provide a directional coupler which couples two orthogonal modes equally. l

.It is still another object of this kinvention to provide a directional .coupler in which the amount of coupling per 3 unit length is relatively large for either of two orthogonal modes.

The novel features .which are Abelieved to.be characteristic of this invention, both as to its organizationand i method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing in which:

Fig. 1 is a perspective view of a directional coupler according to this invention;

Fig. 2 is a section of Fig. 1 taken on line 22 of Fig. l;

Fig. 3 is a sectional view taken on line 3-3 of Fig. 1 and illustrating, vectorially, the coupling Yof the TEM `mode by means of a pair of v,slots arranged according to this invention; and

Fig. 4 is a sectional view similar to Fig. 3 illustrating, vectorially, the coupling of the TEN mode by means of the same pair of slots shown in Fig. 3.

Referring to the drawing wherein like elements are designated by the same reference characters, and particularly to Fig. 1, the directional coupler illustrated therein comprises two parallel and contiguous waveguide sections, 11 and 12, having a common dividing wall, 13. The waveguide sections, 11, 12, are of square cross-section, and therefore, may be used to transmit circularly or elliptically polarized waves. Waveguides having square cross-sections are preferred, since they are best suited for this purpose. It is to be understood, however, that other types of waveguides may also be used.

As shown in Fig. 2, the common dividing wall, 13, is provided with four identical slots 14, 15, 16 and 17.

v Slots 14 and 15, comprising a first pair, are parallel to,

and symmetrically disposed about the longitudinal centerline of wall 13, indicated at 1S. Slots 16 and 17, comprising the second pair, are also parallel to, and symmetrically disposed about this centerline, 18, and are spaced from each other a distance equal to the spacing of the first pair. The lines 19 and 20, intersecting the centers of each of these pairs are separated by a distance of approximately one-quarter waveguide wavelength.

The dimensions of the slots are not critical and do not relate to this invention. Although rectangular slots with rounded corners have been illustrated, slots of difierent shape, such as dumbbell shaped slots, may also be used. Also, the amount of coupling produced by slots 14, 15, 16 and 17, may be increased by providing a plurality of groups of slots, longitudinally arrayed along the common Wall, 13, each group comprising four slots arranged as illustrated in the drawing.

In operation, slots 14, 1S, 16 and 17 are excited by electromagnetic energy propagating in both the T E01 and TEN modes. In Fig. 3, the first pair of slots, 14 and 15, is assumed to be excited by energy propagating in the TEM mode, that is by a horizontally polarized wave propagating in waveguide section 11, and illustrated in terms of its electric vector E. Due to the symmetricalor balanced arrangement of the directional coupler of the invention, each of these slots excites, in waveguide section 12, horizontally and vertically polarized electromagnetic fields, which are illustrated by means of horizontal and vertical electric vectors E1 and E2, respectively. However, Fig. 3 illustrates, vectorially, that whereas the horizontally polarized fields. shown by vector El, excited in waveguide section 12 by slots 14 and 15, respectively, are of equal amplitude and phase, the vertically polarized fields, shown by vector E2, which are similarly' excited, are of equal amplitude but opposite phase. Consequently, the resultant electromagnetic field in waveguide section 12, represented by electric vector E0, is horizontally polarized; and therefore energy is propagated in waveguide section 12 in the T E01 mode only.

Referring to Fig. 4, slots 14 and 15 are assumed to be excited by electromagnetic energy in the TEN mode, that is, by a vertically polarized wave propagating in waveguide section 11, and illustrated in terms of its electric vector E.

Each of these slots excites, in waveguide section 12, horizontally and vertically polarized electromagnetic fields, which are illustrated by means of horizontal and vertical electric Vectors E3 and E4, respectively. However, Fig. 4 illustrates, vectorially, that the horizontally polarized fields (represented by vector E3), cancel in the same manner as do the vertically polarized fields in Fig. 3. Consequently, the resultant electromagnetic field in waveguide section 12, represented by electric vector E0, is vertically polarized, and therefore energy is propagated only in the TEM, mode. It may be noted, in Figs. 3 and 4, as a matter of interest, that horizontal and vertical electric vectors, E0 in waveguide 12, are advanced, and 90 retarded in time phase, with respect to horizontal and vertical electric vectors, E, in waveguide 11 respectively.- a

In the event that this directional coupler is to be used with both TEM and TEN modes, propagating simultaneously, that is, with a circularly or elliptically polarized wave, it may be important that both of these modes be equally coupled from one waveguide section to the other. It has been found that equal coupling of both TEM and TEN modes can be accomplished by a particular spacing of the slots which comprise a given pair. The exact spacing necessary to accomplish this result will depend on the configuration and size of the slots, and the waveguide sections, relative to the operating frequency, and can best be determined experimentally.

Since slots 16 and 17, comprising the second pair, are positioned about the centerline 18, of the common wall 13, of Fig. 2, in the same manner as are slots 14 and 15, comprising the first pair, the coupling provided by both pairs is identical. C0nsequently, energy propagating in one direction, in waveguide section 11, is effectively coupled to waveguide section 12 by two equal coupling links, one-quarter wavelength apart. The distances traveled by this energy, through each of the two coupling links, and down waveguide section 12, in the reverse direction, differ by one-half wavelength, so that cancellation of the energy occurs. That is, the portion of the energy propagating in waveguide section 11, and coupled into waveguide section 12 by slots 14 and 15, is cancelled by an equal lamount of energy also coupled into waveguide section 12 by slots 16 and 17. With respect to energy propagating in the same direction, in waveguide sections 11 and 12, the distances traveled through each of the two coupling links are identical. Thus, the

l energy coupled by slots 14 and 1S adds with the energy coupled by slots 16 and 17, and as a result, energy is transmitted in waveguide section 12 in this direction only. Similar operation is obtained irrespective of which waveguide section is energized, owing to the symmetry of the directional coupler of this invention.

What is claimed as new is.

A directional coupler for transferring electromagnetic wave energy of any predetermined polarization from one waveguide to another waveguide so as to launch in the other waveguide wave energy having substantially the same polarization as said electromagnetic wave energy comprising: two parallel and contiguous sections of square waveguide having substantially identical cross-sections and being of a character adapted to vpropagate orthogonally polarized modes of electromagnetic waves, said Waveguide sections having a common dividing wall having a longitudinal axis, said divding wall being relieved to form a first and second pair of longitudinal slots, the line joining the centers of said first pair of slots and the line joining the centers of said second pair of slots being bisected at right angles by said longitudinal axis of said dividing wall, said line joining the centers of said first pair of slots being separated from said line joining the centers of said second pair of slots by a distance equivalent to odd integer multiples of one-quarter waveguide wavelengths measured along said longitudinal axis.

References Cited in the le of this patent UNITED STATES PATENTS 2,445,348 Ford `luly 20, 1948 2,512,191 Wolf June 20, 1950 2,585,173

6 2,684,469 Sensiper July 20, 1954 2,701,341 Bowen Feb. 1, 1955 2,709,241 Riblet May 24, 1955 OTHER REFERENCES Publication I: A Precision Directional Coupler Using Multi-Hole Coupling, Hewlett Packard Journal, vol. 3, No. 7-8, pub. March-April 1952; HP Co., 395 Page Mill Road, Palo Alto, California. Copy in Div. 69, 178-44 Riblet Feb. 12, 1952 10 1111 U. S. DEPARTMENT OF COMMERCE PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO. 2,817,063 December 1'7, 1957 Louis A. Kurtz It is hereby certified that error appears n the printed specification of the above numbered patent requiring correction and that the said Let sers Patent should read as corrected below.

Column 2, line l., for "special" read spaoal-; line 18, for "it this" read "if" Jthis".

Signed and sealed this '25th day of February 1958.

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Atteeting Officer p Y Commissioner of Patents 

